EP0231980B1 - Method for transmitting measuring signals of at least two sensors via an opticaltransmission link - Google Patents

Description

The invention relates to a method for transmitting at least two measured values by means of light pulses directed from an optical transmitter over an optical transmission path to an optical receiver, the time interval between which is evaluated as a measure of the measured value. A method of this type is known from EP-A1 00 75 701.

Optical transmission links and in particular optical fibers (LWL) are insensitive to electromagnetic interference. They are suitable for use in potentially explosive environments and enable measured values to be transmitted over long distances.

In the known case, the possibility of transmitting optical signals in the form of pulse sequences in which the pulse phase is modulated is already mentioned without further explanation.

Energy from a voltage source is required for the electronic processing of the measured values and for the formation of the optical transmission pulses. In the known case, the required voltage is generated by photo elements, to which light output is supplied via an optical line. Instead, batteries could also be provided in the measuring device. In any case, it is desirable to keep the energy consumption of the measuring device low.

The invention is therefore based on the object of designing the method of the type mentioned at the outset in such a way that the energy consumption for the optical transmission of the measured values is reduced.

The solution is achieved in that the measured values are transmitted directly in cyclical succession in always the same order, that an optical measurement pulse is transmitted for each measured value, the time interval from which an optical measurement pulse assigned to a previous measured value forms a measure of the size of the measured value, and that for each Cycle of measured values an optical identification pulse is transmitted, the time interval from a previous optical measurement pulse is smaller than the minimum possible time interval between two successive optical measurement pulses.

The invention is based on the knowledge that the majority of the energy required is used to form the optical signals. It was recognized that considerably less energy is required for the pulsed transmission which is already considered as an alternative in the known case than for transmission by means of modulated continuous light. In addition, the measurement information during pulse transmission cannot be falsified by variable attenuations of the transmission path.

The fact that, according to the invention, only a single optical pulse is transmitted per measured value and only a single identification pulse is required for a group of several measured values, there is a very low energy consumption which is made available by a single lithium battery over the entire service life of a measuring device can.

The optical transmission path could be a free beam path. However, a single optical fiber is preferably used, by means of which the measured values are transmitted in chronological succession. One of the measured values is identified in the receiving device with the aid of the identification pulse.

No identification is required for the remaining measuring pulses, since they are transmitted in a constant, cyclical sequence to the marked measured value. According to the invention, no additional transmission time is required for the identification pulse.

If measured values have to be transmitted, which can be in the range from zero to a maximum value, it is advantageous that two successively transmitted optical measuring pulses are sent at a time interval t _o + t _n ', the constant time t _{o being} greater than t _k and the time t _n 'depends on the measured value. Then the minimum possible length of time between successive measurement pulses is certainly greater than the time interval between an identification pulse and the previous measurement pulse, so that a clear identification of the identification pulse can always be achieved at the receiving end.

A preferred embodiment of the invention is characterized in that the original measured values are converted into electrical square-wave signals, the duration of which depends in a predetermined manner on the size of the measured values, so that the start of the square-wave signal of the subsequently measured value is initiated by the termination of each square-wave signal and that an identification signal with a constant delay time compared to the start of a square wave signal associated with a predetermined measurement signal is generated. The square-wave signals can subsequently be converted into needle pulses by differentiating stages, which are supplied as a sum signal to a control stage of an LED via a common line.

A preferred embodiment of the invention, for which only a small amount of electronic circuit elements is required, is characterized in that the needle-pulse-shaped electrical output signals of the optical receiver, if necessary after amplification are converted by means of a flip-flop into square-wave pulses, the duration of which is greater than the delay time t _{k of} the identification pulse and less than the difference between a minimum possible measurement time t 1 or t 2 and the delay time t _k that these square-wave pulses are fed to a first input of a first AND gate are, while the input signal of the flip-flop is fed to the second input of the first AND gate, so that in-phase signals are generated at the output of the first AND gate, and that the inverted output signal of the flip-flop and the input signal of the flip-flop to a second AND gate are supplied, at the output of which successive signals are generated in accordance with the time interval between the measuring pulses.

Figur 1

zeigt ein Ausführungsbeispiel einer zur Ausübung des erfindungsgemäßen Verfahrens geeigneten sendeseitigen Schaltung.

Figur 2

zeigt an bezeichneten Stellen der Figur 1 meßbare charakteristische Signalverläufe.

Figur 3

zeigt ein Ausführungsbeispiel einer zur Ausübung des erfindungsgemäßen Verfahrens geeigneten empfangsseitigen Schaltung.

Figur 4

zeigt an bezeichneten Stellen der Figur 3 meßbare charakteristische Signalverläufe.

The invention is explained in more detail on the basis of the description of an advantageous exemplary embodiment shown in the drawing.

Figure 1

shows an embodiment of a transmission-side circuit suitable for carrying out the method according to the invention.

Figure 2

shows measurable characteristic signal curves at designated points in FIG.

Figure 3

shows an embodiment of a receiving circuit suitable for carrying out the method according to the invention.

Figure 4

3 shows measurable characteristic signal profiles at designated points in FIG.

The sizes of two measured values m1 and m2 (eg pressure values) are scanned by means of the capacitive sensors 1 and 2 by means of the transmission device shown in FIG.

The transmitting device finally forms optical pulses 3 from the determined capacitance values of the sensors 1 and 2, which are passed into the optical fiber and transmitted to the receiving device shown in FIG. 3.

A capacitive pressure sensor consists, for example, of a cylindrical base body, on the end faces of which metallized membranes are arranged, the spacings of which from counter electrodes change depending on the pressure.

The transmitting device consists of an oscillator stage 4, a differentiating and decoding stage, which contains the differentiating stages 5, 6 and 7 and a monostable flip-flop 8, and an optical transmitting stage with a light source, which preferably consists of a semiconductor laser diode 10.

Rectangular pulses a and b are generated by the oscillator stage 4, as shown in FIG. 2 as a function of time. The time length t₁ of each pulse a contains the information about the size of the capacitance of the sensor 1 and thus about the measured variable m₁. The time length t₂ of the pulse b is a measure of the measured value m₂.

It is essential that the pulse times t 1 and t 2 follow one another immediately without a break. This is effected by means of the oscillator stage 4 with the electronic switching means known to the person skilled in the art and only indicated in the drawing (cf. the older application P 35 28 416.1). Here, mutually advancing monostable multivibrators are primarily used, the switching state duration of which depends on the charging times of the sensor capacities 1 and 2. The rectangular pulses a and b are converted into needle current pulses by the differentiating stages 5 and 7 (see FIG. 2 pulse sequences d and e).

In addition, from the monostable flip-flop 8, a rectangular pulse sequence c (Figure 2) is passed to the differentiating stage 6, at whose output a current pulse sequence f of Figure 2 erscheint.Der start of pulses c at the time t _k with respect to the beginning of the to be identified Impulse b delayed. The delay time t _k can be predetermined by the capacitance C 1 and the resistor R 1. It must be less than the minimum possible value of the times t 1 and t 2.

und ${\text{t₂ = t}}_{\text{o}}\text{+ t₂′.}$

Bei einer beliebigen Anzahl von zu übertragenden Meßwerten gilt ${\text{tn = t}}_{\text{n}}{\text{′+t}}_{\text{o}}\text{.}$

Dabei enthält die Zeit t_n′ jeweils die Information über den n-ten Meßwert.So that such measured values are measurable and transferable, which can vary from zero to a maximum value, the times t 1 and t 2 (generally t _n ) consist of a measured value-independent constant time t _o plus a measured value-dependent variable time t 1 ′ or t 2 ′ (general t _n ′). So is: ${\text{t₁ = t}}_{\text{O}}\text{+ t₁ ′}$

and ${\text{t₂ = t}}_{\text{O}}\text{+ t₂ ′.}$

The following applies to any number of measured values to be transmitted ${\text{tn = t}}_{\text{n}}{\text{′ + T}}_{\text{O}}\text{.}$

The time t _n 'contains the information about the nth measured value.

In the present case, the fixed time t _o results from the fact that the capacitances of sensors 1 and 2 already have a finite value if the measured variables m1 and / or m2 have the value zero.

The diodes D1, D2 and D3 suppress negative signals, so that a voltage sum signal according to g according to FIG. 2 is present at the control connection of the electronic switch 9, which voltage voltage consists of the sum of the needle-shaped signals d, e and f. In the presence of these needle-shaped signals, the light source is connected to the DC voltage U via the switch 9. As a result, the light source 10 forms optical needle signals 3, which have the chronological sequence of the signals g according to FIG. 2. The capacitor C2, which had previously been charged via the charging resistor R2, is discharged very quickly via the light source.

The short-term but high current flow through the light source then generates an optical pulse. During the long pause between two successive pulses, C2 can then recharge via R 2. The average power consumption is low because the light source is only connected for a short time. The peak current required to generate high optical pulses is taken from capacitor C2 to reduce the load on the voltage source.

While the light source is operated by the unregulated voltage of a lithium battery, a voltage Uo which is constantly regulated by a circuit, not shown, is required for the power supply of stages 4 to 8. Overall, only an average current of about 30 uA is required to power the entire transmitter.

The optical pulse signals 3, which run according to g in FIG. 2, are passed to the photodiode 11 of the receiving device shown in FIG Signals a and b correspond to Figure 2. The output signals o and p are fed together with intermediate signals k and l to an evaluation circuit (not shown), at whose output a DC voltage proportional to the difference between the measured values (m1-m2) is then output. Such an evaluation circuit can be constructed in a manner known to those skilled in the art, for example in the manner described in the earlier application P 35 28 416.1. In this way, for example, a pressure difference of a differential pressure sensor can be read directly.

The photodiode 11 of the photo amplifier 12 is followed by a current-voltage converter. The photodiode 11 generates an electrical current from the optical signal, which then appears as a voltage signal at the output of the operational amplifier OP1. This signal h is amplified again with the operational amplifier OP2. In addition, the DC signal is separated in this stage with the capacitor C3 so that the dark current of the photodiode 11 and offset currents of the operational amplifier OP1 have no influence. The signal is limited with the Zener diode D4 so that no overdrive can occur. The comparator K then generates a TTL-compatible pulse signal i (see FIG. 4). A reference voltage is generated with the resistors R3 and R4. The comparator K switches when the input signal is larger than the reference signal. Interference signals contained in the signal h which are smaller than the reference signal are thus suppressed.

The needle pulse-shaped output signal of the comparator K now passes through the decoding circuit 13. There, the rectangular original signal is regenerated from the needle pulses. The pulse diagram of this stage is shown in FIG. 4. The monostable flip-flop reacts to the falling edge of the needle pulses i of the comparator K. The pulse time t _{m of} the monostable flip-flop MFF must be greater than the time t _k and less than the time t 2. The non-inverted output signal of the monostable flip-flop MFF reaches the gate U1, to which the needle pulse signal is also present. The gate U1 then ensures that only the additional pulse of the needle pulse signal is passed on. This signal then reaches the reset input of the D flip-flop DFF. The inverted output signal of the monostable flip-flop MFF reaches the gate U2. At its output the needle pulse signal appears without the additional pulse. This signal is now sent to the "clock" input of the D flip-flop, which works as a bistable flip-flop, which means that it jumps with every needle pulse. A square-wave signal then appears at the output Q of the D flip-flop, the pulse time being high corresponding to the pulse time t 1 and thus the sensor capacitance C1 and the pulse time at low level corresponding to C2. The correct assignment takes place with the additional pulse, which was blanked with the gate U1 and is at the reset input of the D flip-flop D-FF. This pulse appearing in time t 2 forces a reset of the D flip flop, so that a low level appears at output Q during this time.

The invention was explained on the basis of the description of a transmission of only two measured values for simplicity of illustration. An advantageous application example is the pressure difference measurement. In order to save the energy required by the transmitter, it is advantageous not to transmit the value of the pressure difference directly, but rather the individual pressure values. Then the energy expenditure for electronic conversion and evaluation of the pressure values for the pressure difference value can be delivered at the receiving end. The circuits shown can be modified in a manner familiar to the person skilled in the art in order to be able to transmit more than two measured values, even then only one identification pulse being required.

Claims (7)

1. A method of transmitting measurement values from at least two sensors by means of light pulses which are applied from an optical transmitter, via an optical transmission path, to an optical receiver and whose distance in time is evaluated as a measure for the measurement value, characterized in that the measurement values (m₁, m₂) are transmitted each time in the same order of succession, directly cyclically following each other, in that per measurement value an optical measuring pulse is transmitted whose distance in time (t₁, t₂) from an optical measuring pulse associated with a preceding measurement value is a measure for the magnitude of the measurement value, and in that for each cycle of measurement values an identification pulse is transmitted whose distance in time (t_k) from an optical measuring pulse associated with a preceding measurement value is smaller than the smallest possible distance in time between two successive optical measuring pulses.
2. A method as claimed in Claim 1, characterized in that two successively transmitted optical measuring pulses are transmitted with a distance in time t₀ + t_n', the constant time t₀ being longer than t_k and the time t_n' being dependent on the measurement value.
3. A method as claimed in Claim 1 or 2, characterized in that the transmission path is constituted by a single optical conductor (LWG), at the beginning of which a fight source, notably a semiconductor laser diode (10), couples in the measuring pulses and the identification pulse for supply to a photodetector (11).
4. A method as claimed in any one of the Claims 1 to 3, characterized in that the original measurement values (m₁, m₂) are converted into electric squarewave signals (a, b) whose duration (t₁, t₂) is dependent on the magnitude of the measurement values in a predetermined manner, in that the termination of each squarewave signal (a and b respectively) initiates the beginning of the squarewave signal (a and b respectively) of the next measurement value measured and in that an identification signal (c, f) is generated with a constant delay t_k with respect to the beginning of the squarewave signal (b) associated with a predetermined measuring signal (m₂).
5. A method as claimed in Claim 4, characterized in that the squarewave signals (a,b,c) are converted into needle pulses (d,e,f) by differentiating stages (5,6,7).
6. A method as claimed in Claim 5, characterized in that the needle pulses (d,e,f,) are applied, via a common conductor, to a driver stage (9) of an LED or of semiconductor diode (10).
7. A method as claimed in any one of Claims 1 to 6, characterized in that the electrical output signals in the form of needle-shaped pulses of the optical receiver (12) are converted by means of a multivibrator stage (MFF) possibly after amplification, into square-wave pulses whose duration t_m is longer than the delay t_k and shorter than the difference between a shortest possible measuring time t₁ or t₂ and the delay t_k, in that these squarewave pulses (k) are applied to a first input of a first AND-gate (U₁) whereas the other input of the first AND-gate receives the input signal (i) of the multivibrator stage (MFF), so that signals (m) having the same phase as the identification pulses (f) appear at the output of the first AND-gate (U₁) and in that the inverted output signal (1) and the input signal (i) of the multivibrator stage (MFF) are applied to a second AND-gate (U₂) at the output of which successive needle signals (n) appear in conformity with the distance in time between the measuring pulses.

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